1. Field of the Invention
The invention generally relates to a frequency error correction unit and method, and in particular to a receiver in a wireless local area network (WLAN) communication system.
2. Description of the Related Art
In a communication system, it is important for a receiver to synchronize to the transmitter so that messages can successfully be exchanged between the transmitter and the receiver. In a radio communication system, it is particularly important that a receiver is tuned to the frequency of the transmitter for optimal reception.
A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for a wired LAN. WLAN systems transmit and receive data over the air using radio frequency or infrared technology to minimize the need for wired connections. Thus WLAN systems combine data connectivity with user mobility.
Today most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in a reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
In direct sequence spread spectrum systems, spreading is achieved by encoding each data bit using a code word or symbol that has a much higher frequency and information bit rate. The resultant “spreading” of the signal across a wider frequency bandwidth results in a comparatively lower power spectrum density, so that other communication systems are less likely to suffer interference from the device that transmits the direct sequence spread spectrum signal. Direct sequence spread spectrum employs a pseudo random noise code word known to the transmitter and receiver to spread the data. The code word consists of a sequence of “chips” that are multiplied by (or exclusive-ORed) with the information bits to be transmitted. Many wireless networks conform the IEEE 802.11 standard which employs the well-known Barker code to encode and spread the data. The Barker code word consists of a predefined sequence of eleven chips. One entire Barker code word sequence is transmitted in the time period occupied by an information-containing symbol.
To allow higher data rate transmissions, the IEEE 802.11 standard was extended to IEEE 802.11b. In addition to the 11-bit Barker chips, the 802.11b standard uses an 8-bit complementary code keying (CCK) algorithm for high data rate transmission.
The data transfer rate may also be improved above the symbol rate by employing higher order modulation techniques, including quadrature phase-shift keying (QPSK) modulation. According to such modulation techniques, each bit is represented by a higher number of possible phases. The transmitter therefore generates two signals, the first signal called the “in-phase” (I) signal or “I channel” and a second signal called the “quadrature” (Q) signal or “Q channel” for a 90° phase-shifted sinusoidal carrier at the same frequency.
The IEEE 802.11 standard for wireless LANs using direct sequence spread spectrum techniques employ a training preamble to train a receiver to a transmitter. Each transmitted data message comprises an initial training preamble followed by a data field. The preamble includes a sync field to insure that the receiver can perform the necessary operations for synchronization. For the preamble length, two options have been defined, namely a long and a short preamble. All compliant 802.11b systems have to support the long preamble. The short preamble option is provided in the standard to improve the efficiency of the networks throughput when transmitting special data such as voice and video. The synchronization field of a preamble consists of 128 bits for a long preamble and 56 bits for a short preamble.
A receiver detects the synchronization symbols and lines the receivers internal clock to the symbols in the synchronization field in order to establish a fixed reference time frame with which to interpret the fields in the transmission frame structure following the preamble. The preamble, including the synchronization field, is transmitted at the start of every message (data packet).
When operating a wireless LAN receiver, code synchronization is necessary because the code is a key to despreading the desired information. A good synchronization is achieved when the coded signal arriving at the receiver is accurately timed in both its code pattern position and its rate of chip generation.
The oscillators in a receiver and a transmitter may provide different frequencies due to manufacturing imperfections, different temperatures etc. which result in a frequency drift of the baseband signal. Such frequency differences or frequency offsets are corrected by frequency error correction units at the receiver's site.
Referring now to FIG. 1, frequency synchronization is acquired by repeatedly obtaining a frequency error estimation and performing a frequency error correction of the input signal based on the obtained frequency error estimation. Such a correction procedure is repeatedly performed in a feedback loop.
A frequency error correction unit performing the frequency error correction as shown in FIG. 1 is described with reference to FIG. 2. The frequency error correction unit is a processing loop used to acquire frequency synchronization for the received signal. The received signal 200 is applied to a mixer 210. The output of mixer 210 is fed to a frequency error detector in order to compute the frequency error. The output of the frequency error detector 230 is processed through a loop filter 240 having a predefined filter function and a numerically controlled oscillator (NCO) 250. The output of the numerically controlled oscillator (NCO) is also applied to the mixer 210 to complete the frequency loop to correct for the detected frequency error.
Frequency error correction units still have a number of problems. One problem is that frequency error correction units need a time consuming number of iterative steps to achieve frequency synchronization between the receiver's clock and the input signal. Further, a first frequency error correction is only possible after completing the feedback loop.